From Norwegian patent no. 164 846 there is known an electrically insulated supply tube for admixtures, which is provided centrally in an internal electrode in a plasma torch designed for submersion in a metallurgical smelt.
In U.S. Pat. No. 4 122 293 there is described an external liquid-cooled supply tube for the supply of gas, admixture and electric current to a hollow electrode which is used in an electric arc smelting furnace.
Furthermore, EP 0 178 288 describes a nozzle for a plasma torch specially designed for heating a metallurgical melting pot. The nozzle has an electrode tip attached to a liquid-cooled electrode holder which simultaneously acts as a supply tube for plasma-forming gas and electric current. The electrode tip has a central boring for the plasma-forming gas and the outlet of the boring is designed first as a Laval nozzle and thereafter as a diffuser to permit the gas to be sprayed when it leaves the electrode.
GB 995 152 describes an electric arc torch for a cutting apparatus which emits a jet of gas heated to a very high temperature by means of an electric arc which is struck between a torch body and a workpiece. The torch body consists of one elctrode within an arcing chamber and the exit end of the cutting gas supply pipe can be provided with a venturi nozzle. However, the nozzle is not replaceable.
From U.S. Pat. No. 4 275 287 is known a water-cooled lead-in tube for supply of a reactant to a plasma torch. The lower pare of the lead-in tube is removable in order to facilitate replacement when it is worn after use. However, the lead-in tube is not movable.
During chemical treatment of a reactant, for example during pyrolysis, it is essential that the gas has the correct temperature when it reaches the plasma flame. If the temperature of the gas exceeds a certain value it will react too early. This is undesirable as decomposition products can be formed before the gas reaches the plasma flame, and this can lead to precipitation of such products in the lead-in device and on the electrodes.
It has been found that the known designs of supply devices for gas produce unsatisfactory results when used in a plasma torch which is utilized for chemical treatment of reactant.